Like Spelunky, A Valley Without Wind is a procedurally-generated platformer, meaning levels are algorithmically generated and no two plays will ever be the same. But Valley is more than a platformer; it has elements of crafting, character advancement with a huge number of paths, and an adventure game-like narrative arc.
Once you complete the tutorial, you start in a town surrounded by many available wilderness squares; you select one, and transition to a sidescrolling level, with monsters to fight and with some elements, like trees and rocks, that you can destroy for resources. But within most wildernesses are buildings and dungeons, each a series of levels in their own right, in which you can find resources that provide you with powerups, or that can be used to craft better spells, or to create "guardian powers."
In many buildings, you're given the opportunity to perform a special mission, which can provide more resources, and do such things as increasing the population of your town.
The core grind of the game is leaving town, solving platforming navigation problems and fighting enemies, collecting resources, and bringing them back to town to improve your own spells and other capabilities. Eventually, you can build things in the town that open up other possibilities.
After you have completed five missions, however, the world becomes "tier 2," meaning that monsters are now more powerful -- but of course, by then, you also have access to more powerful spells and capabilities.
As in a Rogue-like, you are likely to die frequently (unless you crank the difficulty down very low), but "dying" is a fairly minor inconvenience; a new "glyphmaster" appears to carry on the game, and your resources and spells are retained. The only real loss is of some character stats, like improved health and mana.
Once you hit tier 5, it is time to take on the climax of the saga, defeating the big evil whatchamacallit -- a common trope, of course.
One nice touch: the difficulty can be tuned on two dimensions -- platforming, and combat. So if, say, you suck at platformers but aren't phased by the combat system (like, ahem, me), you can customize the game to your liking.
On the whole, it's an amazing, genre-blending, noticeably polished game that is quite likely to suck up hours of your time.